Palladium-catalyzed non-directed C[sbnd]H benzoxylation of simple arenes with iodobenzene dibenzoates

Article abstract of DOI:10.1016/j.tetlet.2016.11.055

A palladium-catalyzed non-directed C[sbnd]H benzoxylation of simple arenes with iodobenzene dibenzoates as both benzoxylate source and oxidant has been developed. The catalytic system was greatly promoted by a pyridine ligand. Good functional groups tolerance was showed in both hypervalent iodine reagents and arene substrates, which can be used for synthesis of aryl benzoates through simple aromatic compounds.

Full text of DOI:10.1016/j.tetlet.2016.11.055

Accepted Manuscript
Palladium-catalyzed non-directed C−H benzoxylation of simple arenes with io-
dobenzene dibenzoates
LiLi, Ying Wang, Tingting Yang, Qian Zhang, Dong Li
PII:
S0040-4039(16)31523-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.11.055
TETL 48341
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
8 October 2016
12 November 2016
14 November 2016
Please cite this article as: LiLi, Wang, Y., Yang, T., Zhang, Q., Li, D., Palladium-catalyzed non-directed C−H
benzoxylation of simple arenes with iodobenzene dibenzoates, Tetrahedron Letters (2016), doi: http://dx.doi.org/
10.1016/j.tetlet.2016.11.055
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Tetrahedron Letters
Palladium-catalyzed non-directed C−H benzoxylation of simple arenes with
iodobenzene dibenzoates
Li Li,a, † Ying Wang,b, † Tingting Yang,b Qian Zhang,b Dong Lia, b, 
aHubei Collaborative Innovation Center of Targeted Antitumor Drug, Jingchu University of Technology, Jingmen 448000, China
bSchool of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A palladium-catalyzed non-directed C–H benzoxylation of simple arenes with iodobenzene
dibenzoates as both benzoxylate source and oxidant has been developed. The catalytic system
was greatly promoted by a pyridine ligand. Good functional groups tolerance was showed in
both hypervalent iodine reagents and arene substrates, which can be used for synthesis of aryl
benzoates through simple aromatic compounds.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Palladium catalysis
C–H activation
Benzoxylation
Arenes
Hypervalent iodine reagent
1. Introduction
During the past decade, transition-metal-catalyzed C–H bond
functionalization has emerged as a powerful tool for organic
synthesis. Significant progress have been approached in the
development of transition-metal-catalyzed directing group-
assisted ortho-functionalization of aromatic C–H bond.1 Various
directing groups were developed and employed in the vast
majority of C–H bond functionalizations for promoting both the
reactivity and selectivity.2 However, it also became a main
limitation of these methods as normally additional introduction
and removal procedures of the directing groups were necessary.
The development of transition-metal-catalyzed non-directed C–H
bond functionalization, which avoids prefunctionalization of
substrates and shows great atom economy, is highly desired.3
Among the C–H bond functionalizations, C–O bond formation
especially acyloxylation has attracted considerable attention as
the important role of ester functionality in natural products,
pharmaceuticals, and functional materials.4 In recent years,
significant efforts have been made towards the transition-metal-
catalyzed aromatic C–H acetoxylation with different directing
groups (Scheme 1, (1a)).5 However, the direct C–H acetoxylation
of simple arenes still remained as a challenge. In 1996, a
pioneering work by Crabtree showed the palladium-catalyzed C–
H acetoxylation of arenes with PhI(OAc)2 as oxidant although
both the reactivity and selectivity was far from satisfying.6 Until
Scheme 1. Transition-metal-catalyzed C−H acyloxylation
recently, Sanford discovered that pyridine ligand can promote the
palladium-catalyzed C–H acetoxylation of arenes which could
achieve high reactivity and controlling of the site-selectivity
(Scheme 1, (1b)).7–9 Nevertheless, most of previous research on
C–H bond acyloxylation is focus on acetoxylation, the
benzoxylation of C–H bond is less developed, although aryl
benzoates are also important structures which can be found in
———
† These authors contributed equally
 Corresponding author. Tel.: +86 27 59750460; fax: +86 27 59750482; E-mail:dongli@mail.hbut.edu.cn

2
Tetrahedron
Table 1. Ligand Screeninga, b
many natural products.10 A number of methods were reported
recently for transition-metal catalyzed directing group-assissted
ortho-C–H benzoxylation with various benzoxy surrogates and
external oxidants.11 An only example of non-directed C–H
benzoxylation of simple arenes was reported by Liu in 2015
under palladium catalysis with benzoic acids and iodosobenzene
as oxidant however long reaction time was neccessay and only
electron rich arenes can be employed.12
In 2015 our group reported a facile and efficient palladium-
catalyzed ortho-benzoxylation of 2-arylpyridines with
iodobenzene dibenzoates as both benzoxylate source and oxidant
(Scheme 1, (2)).13 The use of iodobenzene dibenzoates avoided
employment of independent acyloxylate source and oxidant. It
also demonstrated prior reactivity to acetoxylation under the
standard conditions. Inspired by this work and previous study, 7
we envisioned pyridine could also act as independent ligand to
promote the palladium-catalyzed C–H benzoxylation of simple
arenes (Scheme 1, (3)). The behavior of iodobenzene dibenzoates
in this reaction comparing with acetoxylation was also worth to
be studied.
2. Results and discussion
aReaction conditions: Pd(OAc)2 (5 mol%), ligand (5 mol%), iodobenzene
dibenzoate (2a) (0.5 mmol) in benzene (1a) (1.0 mL) stirring at 100 ºC for 4
h. bGC yield by using PhNO2 as an internal standard.
In initial study, the reaction between benzene (1a) and
iodobenzene dibenzoate (2a) with Pd(OAc)2 under 100 ºC
showed poor reactivity (Table 1). However, when pyridine (L1)
was added as the ligand, 28% yield of benzoxylation product 3a
was detected. We subsequently examined various commercial
available pyridine derivatives as shown in Table 1 (L2–L13).
Most mono-substituted pyridines which possessed a methyl-,
methoxyl-, N,N-dimethylamino-, chloro- or acetyl group
provided comparable product yield (26–37%), among which the
4-chloropyridine (L9) gave the best result. However the use of a
steric bulky ligand 4-tert-butylpyridine (L5) lead to a decreased
yield. No significant electronic effect was observed through these
results however steric factor might be more effective. Muti-
substituted pyridines were also investigated. To our delight,
although 3,5-dichloropyridine (L11) didn’t generate better result,
2,4-dimethylpyridine (L12) and 2,4,6-trimethylpyridine (L13)
improved the product yield to 43% and 44% respectively.
Another special pyridine analog ligand L14 which has shown
Table 2. Optimization of the reaction conditionsa
Benzene
(mL)
Yield
(%)b
29
Entry
Pd(II)
Pd/ Ligand
1
2
PdCl2
Pd2(dba)3
1/1
1/1
1
1
19
3
4
5
6
7
8
Pd(OCOCF3)2
Pd(OCOCF3)2
Pd(OCOCF3)2
Pd(OCOCF3)2
Pd(OCOCF3)2
Pd(OCOCF3)2
Pd(OCOCF3)2
Pd(OCOCF3)2
Pd(OCOCF3)2
1/1
1/1
1/1
1.5/1
1/1.5
2/1
1/2
1/1
1
2
0.5
1
1
1
1
1
1
53
50
13
48
40
14
<5
58
60
particular
efficiency
in
various
Pd-catalyzed
C–H
functionalization14 was also examined but unfortunately it was
not suitable in this reaction.
9
10c
11d
1/1
aReaction conditions: Pd(II) (5 mol%), L13, iodobenzene dibenzoate (2a) (0.5
Further optimization was carried out by using 2,4,6-
trimethylpyridine (L13) as the ligand (Table 2). Other Pd(II)
catalysts were subsequently examined. Although PdCl2 and
Pd2(dba)3 didn’t provide better result than Pd(OAc)2 (Table 2,
entries 1 and 2), 53% product was generated when Pd(OCOCF3)2
was used (entry 3). Effect of concentration was checked by
altering the amount of benzene. Diluted reaction resulted in a
slightly lower product yield (entry 4) however more concentrated
condition was not favorable which lead to a great yield
decreasing (entry 5). The Pd/ligand ratio was also investigated.
Changing the ratio from 1:1 to 1.5:1 or 1:1.5 resulted in lower
product yields. Further alteration such as Pd/ligand 2:1
drastically decreased the reactivity while the 1:2 combination
even resulted in almost no reaction (entries 6–9). The 1:1
Pd/ligand ratio was necessary for high efficiency because it
would generate a coordinatively unsaturated Pd-pyridine species
which is essential for the subsequently C–H bond activation. This
outcome was coincident with Sanford’s previous study.7 Finally,
we found further improvement could be achieved by increasing
the catalyst loading to 10 or 15 mol%, which provide 58 and 60%
yield of product respectively (entries 10 and 11).
b
mmol) in benzene (1a) stirring at 100 ºC for 4 h. GC yield by using PhNO2
as an internal standard. cWith 10 mol% Pd(OCOCF3)2. dWith 15 mol%
Pd(OCOCF3)2.
A preliminary kinetic study was carried out for illustrate the
reaction rate as shown in Figure 1. With
5 mol%
Pd(OCOCF3)2/L13, the reaction proceeded relatively fast during
the first 2 hours and almost completed after 4 hours. Compare
with the previous acetoxylation with same catalyst loading,7 it
demonstrated similar reaction rate however reached the
equilibrium at a lower product yield.
The benzoxylation of benzene (1a) with various iodobenzene
dibenzoate derivatives (2) were investigated under the optimized
conditions as shown in Table 3. Both electron-donating
functional groups such as OMe and Me and electron-withdrawing
functional groups such as F, Cl, Br and NO2 was tolerated on the
benzoyl ring, affording the corresponding benzoxylation products
in moderate to good yields.

Table 3. The palladium-catalyzed benzoxylation of arenesa
Entry
1
Substrate
(1b)
Product
Yield (%)b
60
(o/p=52:48)
(3h)
(3i)
2
3
40
32
(1c)
Figure 1. Plot of reaction rate of bezene benzoxylation. Reaction conditions:
Pd(OCOCF3)2 (5 mol%), L13 (5 mol%), iodobenzene dibenzoate (2a) (0.5
mmol) in benzene (1a) (1.0 mL) stirring at 100 ºC for 4 h. Yields determined
by GC.
(1d)
(3j)
(3k)
(3l)
4
5
6
7
8
9
50
68
56
46
43
33
(1e)
(1f)
(1g)
Table 3. The palladium-catalyzed benzoxylation of bezenea
Yield
Entry
1
Ar
Product
(%)b
(3m)
(3n)
(3o)
(3p)
4-OMe-C6H4
68
(2b)
(3b)
(3c)
(1h)
(1i)
4-CH3-C6H4
2
3
4
5
6
46
73
52
41
55
(2c)
4-F-C6H4
(2d)
(1j)
(3d)
(3e)
(3f)
aReaction conditions: Pd(OCOCF3)2 (5 mol%), L13 (5 mol%), iodobenzene
dibenzoate (2a) (0.5 mmol) in 1 (1.0 mL) stirring at 100 ºC for 4 h. bGC yield
by using PhNO2 as an internal standard.
4-Cl-C6H4
(2e)
4-Br-C6H4
(32–60%). The reaction of toluene gave a nearly 1:1 ratio
mixture ortho- and para-benzoxylation product (entry 1).
However, anisole reacted to generate mainly para-benzoxylation
product in lower yield (entry 3). The reaction of electron-
deficient arenes showed better site-selectivity, providing the
meta-benzoxylation product in moderate to good yields (entries
4–7). Other disubstituted arenes with different type of
substituents such as 2-methyl-iodobenzene (1i) and 2-methoxyl-
bromobenze (1j) were also applicable for the C–H benzoxylation
and gave the desired products in lower yield but good site-
selectivity (entries 8 and 9).
(2f)
4-NO2-C6H4
(2g)
(3g)
aReaction conditions: Pd(OCOCF3)2 (5 mol%), L13 (5 mol%), 2 (0.5 mmol)
b
in 1a (1.0 mL) stirring at 100 ºC for 4 h. GC yield by using PhNO2 as an
internal standard.
The reaction mechanism was subsequently investigated
through intermolecular KIE study (Scheme 2). By using 1:1 ratio
of benzene and d6-benzene as the substrates, the KIE was
determined to be 7.62, which implied that the C−H bond
activation was involved in the rate-determining step (RDS).
This method can also be applied for benzoxylation of other
arenes (1) as shown in Table 4. Various electron-rich or electron-
deficient arenes can be benzoxylated under the standard
conditions. Generally, the electron-deficient arenes such as
chlorobenzene (1e), bromobenzene (1f), iodobenzene (1g) and
dichlorobenzene (1h) gave higher yields (46–68%) than electron-
rich arenes such as toluene (1b), p-xylene (1c) and anisole (1d)
8, 13
Based on the above study and previous reports,7,
a
plausible mechanism for the pyridine ligand-assisted palladium-
catalyzed non-directed C−H benzoxylation of arenes with

4
Tetrahedron
iodobenzene dibenzoate (2a) was proposed in Scheme 3.
Initially,
Scheme 4. Control experiments
equivalent PhI(OCOPh)2 and PhI(OCOCF3)2 generated the
benzoxylation and acetoxylation product in a slightly higher ratio
71:29 (Scheme 4b). These results proved that in the reductive
elimination step benzoxylation was more favored than
acetoxylation. Moreover, it also explained why Pd(OCOCF3)2
was slightly more efficient than Pd(OAc)2 in this method.
3. Conclusions
Scheme 2. Intermolecular KIE study
In conclusion, we have developed a palladium-catalyzed non-
directed C–H benzoxylation of simple arenes using iodobenzene
dibenzoates as both benzoxylate source and oxidant which made
the protocol simple and facile. Pyridine ligand-assisting strategy
was used which greatly promoted the reactivity. Good functional
groups tolerance was showed in both hypervalent iodine and
arene substrates. It would provide a new methodology for
synthesis of aryl benzoates through simple aromatic compounds.
Preliminary mechanistic study was also carried out and a
plausible mechanism was proposed.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (NSFC, No. 21302046), the Hundred-Talent Program
Fund of Hubei University of Technology and Jingchu University
of Technology for financial support.
Supplementary Material
Supplementary data associated with this article can be found,
in the online version, at doi:
Scheme 3. Proposed mechanism of the Pd-catalyzed C−H
benzoxylation of benzene
References and notes
the pyridine interacted with Pd(OCOCF3)2 to form a pyridine-
ligated Pd(II) species A, which was coordinated unsaturated and
subsquently activated the benzene C–H bond to form a σ-aryl
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dibenzoate (2a) to generate an Pd(IV) benzoate complex C (or
bimetallic Pd(III)-Pd(III) complex).15 Finally, reductive
elimination of intermediate C afforded the benzoxylation product
3a rather than acetoxylation and regenerated the palladium(II)
species.
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3213.
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Graphical Abstract
Palladium-catalyzed
non-directed
C−H
Leave this area blank for abstract info.
benzoxylation of simple arenes with iodobenzene
dibenzoates
Li Li,† Ying Wang,† Tingting Yang, Qian Zhang, Dong Li

4
Tetrahedron
Highlights
► A palladium-catalyzed non-directed C–H benzoxylation of simple arenes is developed.
► Hypervalent iodine reagent iobenzene dibenzoates was used as both benzoxylate source and oxidant.
► Pyridine ligand-assisting strategy was used which greatly promoted the reactivity
► Good functional groups tolerance was showed in both hypervalent iodine and arene substrates.